1.Field of the Invention
The present invention is directed to refracting telescopes and more specifically to an annular objective lens for such telescopes.
2. Prior Art
The telescope is an optical system by means of which an enlarged image of a distant object may be viewed. An astronomical refracting telescope consists of two convergent lenses the first of which, the objective, forms a real inverted image which is examined using a second lens called the eyepiece. In the normal state of adjustment the second focal plane of the objective coincides with the first focal plane of the eyepiece so that an incident pencil of parallel rays emerges as a parallel pencil. The image may be erected by the use of an auxiliary lens.
In the astronomical reflecting telescope, the light from a celestial objective is received on a concave primary mirror. The mirror serves the same purpose as the objective in a refracting telescope, namely, to form a real image of the object in its focal plane. This image is either received directly on a photographic plate or is examined visually be an eyepiece. This type of arrangement is the most common form of astronmical telescope used today. However, reflector telescopes pose problems in construction, particularly the problem of laying down a metallic reflecting film over so large an area. Moreover, the films are subject to deterioration and must be replaced periodically.
Although a refracting telescope is much more durable and easier to manufacture the size of the objective lens has always been a drawback. At the famous Yerkes Observatory of the University of Chicago, the telescope objective has a diameter of 40 inches or approximately 100 centimeters. With this telescope, therefore, objects with an angular separation of about one eighth of a second can be resolved. The 40 inch objective has been recognized as the practical limit since beyond 40 inches the objective lens will begin to deform under its own weight and it is very difficult to keep the lens stress free. In conjunction with this circumstance, the effect of inhomogeneities in the glass become increasingly intolerable.
Another drawback in the use of refracting telescopes for deep space observations has been the resolving power. The ability of a telescope to separate two distant objects close together, that is, the resolving power of the telescope, depends on the aperture of the objective. It has been proven that by blocking out the center of an objective lens that the resolving power of the lens will be increased. However, the light gathering capabilities of such a lens are considerably reduced making such a system completely unsuitable for deep space astronomical telescopes.